Volume 16, Number 3
MRI's Better Peek at Plaque
Joe Carstairs' neurologist found him irritating. Not the man himself, but the fact that he'd had two small, separate strokes in brain areas fed by the left internal carotid -- with little narrowing of that artery. It was perplexing. Carstairs' blood pressure was OK, though he loved his french fries. He probably had arterial plaques. But, because the carotid looked wide open, evidence was slim for either that or for plaque-generated atheromas, the complex artery wall lesions that, ruptured, can trigger stroke. What, then, was throwing clots?
"For years, we'd
measure the degree of arterial narrowing to predict risk of stroke," says
neuroradiologist Bruce Wasserman, M.D. "But we've known stenosis
is only part of the picture." Now, by combining slightly modified MRI with
a standard contrast agent, he has a technique that clarifies the plaque
structure of carotid arteries. The details are sharp enough to measure,
Wasserman says, which should greatly improve risk-predicting. Plaques removed during patients' endarterectomies reveal that risky ones develop a thin fibrous outer layer, or "cap," with an inner lipid "core." Studies suggest thinner caps rupture, exposing a powerful blood coagulant in the lipid core-a nursery for a stroke-causing thrombus.
Recently, in patients facing endarterectomy, Wasserman imaged the carotid artery in the neck at the plaque-prone area that branches into the internal and external vessels. He uses "an ordinary MRI, the 1.5 tesla magnet most people do," but he's added a surface coil that acts like a magnifying lens over the body part to be imaged. The coil steps up the signal, giving high-resolution pictures. Added gadolinium, the contrast agent, highlights the fibrous cap. When Wasserman compared before-surgery images with patients' actual plaques afterward, the two corresponded. "This may be the first accurate way to measure fibrous cap thickness in living patients," he says.
His next step is to verify a potentially better predictor of stroke risk: the cap-to-core ratio. Finally, because signs suggest plaque first ruptures where the fibrous cap's inflamed, Wasserman hopes to use his technique to reveal arterial hot spots of inflammation.
And Carstairs? Because he was in a study, Wasserman successively monitored his carotid artery. As suspected, Carstairs' vessel had become plaque-laden and, in the phenomenon of vascular remodeling, its outer wall mushroomed outward, pulling the narrowing artery open for time being. "But it's a situation ripe for a thrombus," Wasserman says, as he showed Carstairs the images, and a powerful way to get a reluctant patient to nix the french fries.
Joe Carstairs is not the patient's name.
For information, call 410-614-9200.